1. Field of the Invention
The present invention relates to an electronic endoscope for observing a target in a side-viewing direction perpendicular to an insertion direction of the electronic endoscope.
2. Description Related to the Prior Art
In the medical field, electronic endoscopes are used in various diagnoses and treatments. The electronic endoscope has an insert section to be inserted into a subject. A distal portion of the insert section is provided with a capturing optical system for taking in image light of a target in the subject and an image sensor for photoelectrically converting the image light taken in by the capturing optical system. Generally, an image capture window of the capturing optical system is disposed on a distal end surface of the distal portion to take in the image light in a front-viewing direction. The front-viewing direction is parallel with an insertion direction of the insert section.
There is a conventional electronic endoscope provided with a mechanism to vary a focal length of a capturing optical system so as to switch the focal length between normal and magnifying observation (for example, see Japanese Patent Laid-Open Publication No. 11-342104). When the normal observation is switched to the magnifying observation in a state that a distal end surface of a distal portion is positioned close to a target, a depth of field of the capturing optical system becomes shallow. As a result, it becomes difficult for the capturing optical system to focus on a target's surface.
To solve the problem, the electronic endoscope disclosed in the Japanese Patent Laid-Open Publication No. 11-342104 is provided with a hood attached to the distal portion of the insert section. The hood is used for ensuring a predetermined distance between the target's surface and the capturing optical system. The size of the hood is determined such that an end surface of the hood coincides with the best focus position of the capturing optical system. With the end surface of the hood coming in contact with the target's surface, the capturing optical system focuses on the target's surface.
When the electronic endoscope is inserted into a tract with a small diameter, for example, esophagus, it is difficult to change a direction of the distal portion. In this case, an electronic endoscope allowing observation in a radial direction (side-viewing direction) orthogonal to the insertion direction is used (see Japanese Patent Laid-Open Publications No. 2010-12079, No. 61-261713, No. 10-311954, and No. 3-269407). The electronic endoscopes disclosed in the Japanese Patent Laid-Open Publications No. 2010-12079 and No. 61-261713 allow observation in both the front-viewing and side-viewing directions. First, the endoscope is inserted into the subject and the observation in the front-viewing direction is performed. When an abnormal region is found, a side-viewing capturing optical system is brought close to the abnormal region to perform the observation in the side-viewing direction.
In the magnifying observation using the side-viewing capturing optical system of the endoscope disclosed in the Japanese Patent Laid-Open Publications No. 2010-12079, No. 61-261713, No. 10-311954, or No. 3-269407, it is difficult to focus on the target's surface when the distal end portion gets too close to the target's surface. To solve the problem, the Japanese Patent Laid-Open Publication No. 11-342104 discloses the hood for ensuring a predetermined distance between the capturing optical system and the target's surface as described above. However, the thickness of the hood increases the outer diameter of the distal portion. Additionally, to ensure a desired observation area of the side-viewing capturing optical system, the size of the hood needs to be increased in its radial direction. Thus, the hood increases the size of the distal portion.